1. Field of the Invention
This invention relates to a curable resin of the polyimide type adapted for use as an insulating protective coating on electronic parts and a process for preparing the curable resin. It also relates to an electronic part protective coating obtained by curing the resin.
2. Prior Art
Polyimide resins having heat resistance and improved electrical and mechanical properties have been utilized as insulating protective coatings for electronic parts. Since most polyimide resins are insoluble in organic solvents, polyimide protective coatings are generally prepared by using a solution of polyamic acid which is a polyimide precursor, and applying the solution to substrates, followed by heat curing. This conventional approach, however, has several problems. For example, the polyamic acid solution has too high viscosity to work with and requires high temperatures above 300.degree. C. for heat curing. Cured polyimide resin coatings show poor adherence to such substrates as nickel, aluminum, silicon, and silicon oxide film.
Many proposals were made for improving the adherence to substrates. Japanese Patent Publication (JP-B) No. 27439/1968 which corresponds to U.S. Pat. No. 3,325,450 and No. 7213/1984 which corresponds to Great Britain Patent No. 1,571,999 propose polyimide-siloxane copolymers in which a diamine component, which is a reactant for forming polyimide, is partially replaced by a siloxane-containing diamine. Also, JP-B 32162/1983 (U.S. Pat. No. 3,950,308), 32163/1983 (U.S. Pat. No. 3,926,911), 266436/1986, 207438/1986 (U.S. Pat. No. 4,672,099), and 29510/1989 discloses methods of mixing or reacting a polyamic acid or polyimide precursor with a silane having an amino group or acid anhydride group. However, the former method had the problem that copolymers showed poor heat resistance due to an increased siloxane content. In the latter method, the polyamic acid solution experienced a substantial loss of shelf stability with an increasing amount of silane added.
It is also proposed to react polyamic acid or polyimide oligomer with an amino group-containing alkoxysilane as disclosed in Japanese Patent Application Kokai (JP - A) Nos. 157427/1981, 157428/1981, 240730/1985 (U.S. Pat. No. 4,609,700) and 266436/1986. These methods also have the problem that the amount of silane added for shelf stability improvements is limited.
For avoiding heat treatment at high temperatures above 300.degree. C. required for curing polyamic acid to form polyimide coatings, it is proposed to dissolve a polyimide resin having a siloxane linkage in a solvent as disclosed in JP-A 83228/1986, 118424/1986 (U.S. Pat. No. 4,586,997) and 118425/1986 (U.S. Pat. No. 4,701,511). Polyimide resin coatings obtained by these techniques are less solvent resistant in nature and thus unacceptable in some practical aspects.